So I just noticed that in iSEM, in AU mode, as there's no keyboard view, that there is no mono/poly button... Hmmm... I've poked everywhere... Where the F is Waldo?!!!
I have this same issue - I can only find it in stand-alone.
Inquiring minds want to know
There's no way to change the state in the AU, however, if you save a patch as mono or poly in the standalone version, it will retain that state in the AU.
There are a few other issues with feature parity between standalone and AUv3 versions noted here.